[3] I.G. Wright, P.J. Maziasz, F.V. Ellis, et al. MATERIALS ISSUES FOR TURBINES FOR OPERATION IN ULTRA-SUPERCRITICAL STEAM. Oak Ridge National Laboratory
Philo 6 Philo 6 was the first supercritical coal fire plant in the world, commissioned in 1953 and built in 1957. It ran until 1975 when the Clean Air Act was passed and plant managers determined the cost of the environmental upgrades exceeded the plant’s worth. Philo 6 was built as a prototype plant, only large enough to test the idea of supercritical coal power plants. The 125 MW plant operated at 621 C and 31 MPa. The cost of building the Philo 6 plant was $20.3 million, in terms of its capacity cost $170 per kW. This capital cost is more than 20% greater than the cost of building a traditional subcritical unit in that time. However, the engineering challenges of building Philo 6 included many more factors than developing high temperature and pressure materials. The physical difference between boiling water and changing phases at supercritical conditions presented issues such as the development of a novel “steam generator” as opposed to a boiler and the unforeseen hurdles of water purification.
Eddystone Station
The Eddystone Station is currently the highest temperature and pressure coal fire plant operating in the US. It was commissioned in 1954, built in 1960, and anticipated to retire before 2025. The Eddystone unit is larger than the Philo 6 with a capacity of 325 MW, but is relatively small for coal fire power plants due to limitations of manufacturing the special alloy rotor in the 50s. The Eddystone station operates at 650 C and 34 MPa. Materials selection was based on a comprehensive study of stainless steels and super strength alloys used in applications such as high temperature gas turbines. Considerations included mechanical and physical properties, manufacturing and fabrication ability, availability, cost, weld ability, and response to heat treatments. At the time the plant was designed, behavior of steam at extreme temperature and pressure conditions were unknown, so engineers extrapolated data from available steam tables and created “best guess” prediction values for the conditions to be experienced in the turbine. The turbine was manufactured by Westinghouse with components made of a number of different steels:
Nozzle blocks, inner cylinder, diaphragm, vanes: 316 stainless steel*
Rotor: Discalloy
Blades: K42B Bolting: W-545 Outer casting: 2.25Cr-1Mo steel *see reference for alloy compositions After a few years of operation, SP nozzle block experienced severe erosion, SP inner shell distorted due to the inadequate thermal properties of austenitic steel, SP rotor was replaced with a new A286 alloy after alloying element segregation of the original discalloy material, and copper deposits on SP blades occurred as a result of Cu ion release into feedwater from Cu alloyed steel pipes. Modifications and replacements were made to mitigate these issues and continue function of the Eddystone station. Although the Eddystone station was generally successful, it was not economically beneficial and all future supercritical coal power plants built in the US were designed for operation at much more moderate conditions. Limitations in Supercritical Technology Typical US supercritical plants operate at 566 C and 24 MPa. The low cost of coal in the US is a major disincentive to operating at higher conditions. With higher fuel prices in Japan and Europe, development of ultra-supercritical plants is of a higher priority. Inherent limitation of high temperature steels is 620 C where martensitic-ferric steels cannot be used. Austenitic Ni-based steels are needed, although they lack adequate mechanical properties at these temperatures. It is recommended that the inner cylinder and steam chest are manufactured from the same material as the rotor, to avoid distortion and thermal mismatch. The HP rotor and discs experience the most extreme conditions and likely require a Ni-based alloy. The IP rotor experiences lower pressure, but is most susceptible to oxidation corrosion. In addition to material mechanical and physical properties, major factors in material selection also include the ability to cast and form appropriate size and shape components, and the ability to weld materials to one another. Advancements in material performance and life cycle analysis need to be made for more accurate behavior prediction.
[2] ASME, Advancement of a technology: Philo 6 Steam – Electric Generating Unit
[3] I.G. Wright, P.J. Maziasz, F.V. Ellis, et al. MATERIALS ISSUES FOR TURBINES FOR OPERATION IN ULTRA-SUPERCRITICAL STEAM. Oak Ridge National Laboratory
Philo 6
Philo 6 was the first supercritical coal fire plant in the world, commissioned in 1953 and built in 1957. It ran until 1975 when the Clean Air Act was passed and plant managers determined the cost of the environmental upgrades exceeded the plant’s worth. Philo 6 was built as a prototype plant, only large enough to test the idea of supercritical coal power plants. The 125 MW plant operated at 621 C and 31 MPa. The cost of building the Philo 6 plant was $20.3 million, in terms of its capacity cost $170 per kW. This capital cost is more than 20% greater than the cost of building a traditional subcritical unit in that time. However, the engineering challenges of building Philo 6 included many more factors than developing high temperature and pressure materials. The physical difference between boiling water and changing phases at supercritical conditions presented issues such as the development of a novel “steam generator” as opposed to a boiler and the unforeseen hurdles of water purification.
Eddystone Station
The Eddystone Station is currently the highest temperature and pressure coal fire plant operating in the US. It was commissioned in 1954, built in 1960, and anticipated to retire before 2025. The Eddystone unit is larger than the Philo 6 with a capacity of 325 MW, but is relatively small for coal fire power plants due to limitations of manufacturing the special alloy rotor in the 50s. The Eddystone station operates at 650 C and 34 MPa. Materials selection was based on a comprehensive study of stainless steels and super strength alloys used in applications such as high temperature gas turbines. Considerations included mechanical and physical properties, manufacturing and fabrication ability, availability, cost, weld ability, and response to heat treatments. At the time the plant was designed, behavior of steam at extreme temperature and pressure conditions were unknown, so engineers extrapolated data from available steam tables and created “best guess” prediction values for the conditions to be experienced in the turbine. The turbine was manufactured by Westinghouse with components made of a number of different steels:
Nozzle blocks, inner cylinder, diaphragm, vanes: 316 stainless steel*
Rotor: Discalloy
Blades: K42B
Bolting: W-545
Outer casting: 2.25Cr-1Mo steel
*see reference for alloy compositions
After a few years of operation, SP nozzle block experienced severe erosion, SP inner shell distorted due to the inadequate thermal properties of austenitic steel, SP rotor was replaced with a new A286 alloy after alloying element segregation of the original discalloy material, and copper deposits on SP blades occurred as a result of Cu ion release into feedwater from Cu alloyed steel pipes. Modifications and replacements were made to mitigate these issues and continue function of the Eddystone station.
Although the Eddystone station was generally successful, it was not economically beneficial and all future supercritical coal power plants built in the US were designed for operation at much more moderate conditions.
Limitations in Supercritical Technology
Typical US supercritical plants operate at 566 C and 24 MPa. The low cost of coal in the US is a major disincentive to operating at higher conditions. With higher fuel prices in Japan and Europe, development of ultra-supercritical plants is of a higher priority. Inherent limitation of high temperature steels is 620 C where martensitic-ferric steels cannot be used. Austenitic Ni-based steels are needed, although they lack adequate mechanical properties at these temperatures. It is recommended that the inner cylinder and steam chest are manufactured from the same material as the rotor, to avoid distortion and thermal mismatch. The HP rotor and discs experience the most extreme conditions and likely require a Ni-based alloy. The IP rotor experiences lower pressure, but is most susceptible to oxidation corrosion.
In addition to material mechanical and physical properties, major factors in material selection also include the ability to cast and form appropriate size and shape components, and the ability to weld materials to one another. Advancements in material performance and life cycle analysis need to be made for more accurate behavior prediction.